Cracking residual oils



Jan. 17, 1939. w. K. LEWIS CRACKING RESIDUAL OILS Filed Nov. l20, v1934wNj Patented Jan. 17,' 1939 UNITED STATES PATENT OFFICE oaAoKING misiDUAL oILs `Application November 20, 1934, Serial No. 753,809

6 Claims.

The present invention relates to cracking residual oils such as, forexample, reduced crudes, and will be understood from the followingdescription when read in conjunction with the drawingthe sole figure ofwhich is a diagrammatic representation partly in section of an`apparatus appropriate for carrying out the invention.

One object of the present invention is to obtain a maximum yield ofnaphtha in the cracking of residual stocks. Another object is toeliminate or at least diminish the amount of coke formation vwhich veryoften accompanies the cracking of such stocks. Still another object isto obtain a fuel oil of required fluidity in the cracking process.

According `to the present invention, the residual `stock is heated to acracking temperature or at least a temperature such that a furthertemperature increase will induce a substantial cracking of the stock.The heated residue is then admixed with a cycle gas oil or itsequivalent which has been heated to a still higher temperature and whichtolerates such higher temperatures due to the absence of tarry matter.The mixture of residual and cycle gas oil is then passed through adigesting Zone, preferably of the type known as a soaking drum, for thepurpose of obtaining an appreciable amount of cracking. Such a processof obtaining additionalY cracking of a residual stock by admixture witha clean cycle gas oil heated to a higher temperature than the residualstock has the disadvantage that the clean circuit synthesizes a tarwhich possesses a certain although low volatility. The more volatilepartof it which willvbe designated as light tar, has a volatility whichis identical with a large part of the heavy gas oil synthesized from theresidue bycracking. However, this heavy gas oil is a premium crackingstock Whereas the light taris a stock whichwhile it will stand quite y ahigh temperature,when it does reach its cracking temperature it goes toinordinate quantities of gas and coke. If recycled in an ordinary unitthis dirty light tar goes either to the residual stream of coil No. 1 orto the clean circuit of coil No. 2. In either case it is bad.y

The heavy gas oil obtained in my process from the residual stock cannotbe separated from the light tar by rectification because they haveapproximately the same volatility. Use of selective solvents or similarmethods is impractical. However, they can be separated by sending thetwo to a third circuit in which the cracking conditions are adequate tocrack the heavy "gas oil but (Cl. 19E-49) inadequate to coke the lighttar. This separation, which represents an important feature of thepresent invention, depends therefore on taking advantage of one propertyin which the components differ significantly; that is the susceptibilityto cracking. The cracking conditions can be varied by either temperatureand/or pressure and time, but are preferably predominantly varied by thetemperature. the third circuit should be lower than in the second andhigher than in the first. Thus, for example, for a specific crude forwhich the coil outlet temperature on the residuum in the first coilshould be G-.850 F. or lower, and on the light gas oil in the secondcoil should be 975- 1000 F. or higher, on the third coil through whichthe mixture of heavy gas oil and light tar is sent, it should be betweenS50-975 F., and preferably between 9D0-950" F. 'Ihese temperatures areillustrative'onlysince residua may be successfully cracked at highertemperatures. It is advantageous in many cases to maintain the outlettemperature of the heating coil for the cycle gas oil at least F. higherthan the outlet temperature of the heating coil for the residuum.

Practically any oil which can be distilled out of a practically oncecracked residuum is a good cracking stock. Hence, the tar from theresiduum stream should be flashed in a flashvaporizer to the heaviestpractical pitch. Such pitch will be too heavy for a fuel oil. However,the light tar from the third circuit is of low molecular weight andthin, although readily cokable because of its low hydrogen-carbon ratio.This light tar is therefore used in the preferred form of my process forfluxing the pitch to a suitable blended fuel.

The cycle stock fed to coil 2 is capable of producing a tar of higher A.P. I. gravity number than that of the tar produced from the cycle stockfed to coil 3. Under the conditions of operation maintained in coil 2and in the reaction chamber 5, however, the tar produced from the cyclestock will be negligible so that, for all intents and purposes, the tardrawn off from fractionating tower Il may be considered the tar producedfrom the residual feed oil fed into coil I. Thus, in order to operatethe unit to produce a tar of specified gravity from a given feed stock,the separating or fractionating tower operating on the productsresulting from the passage of the fresh feed stock through the heatingcoil and reaction chamber is so controlled as to yield a tar of thelowest possible A. P. I. gravity number without coking and this tar isbrought up to the The temperature in f desired gravity number by beingblended with a tar of higher gravity number produced by cracking a gasoil recovered from said separator. By this procedure, the constituentssuitable for cracking in the initial material are utilized to thefullest extent and at the same time a fuel oil of the desiredgravitynumber is obtainable.

The function of coil No. 1 is primarily to crack residual oil into heavygas oil and some light gas oil. The function of coil No. 2 is primarilyto crack light cycle gas oil to naphtha. The function of coil No. 3 isprimarily to make it possible to separate heavy gas oil synthesized incoil No. 1 and the soaker following it from the light tar of the samevolatility synthesized in coil No. 2 by cracking the former preferablypredominantly to light gas oil. The function of the flash vaporizer isto strip out of the cracked residuum all except the completelynon-volatile fractions. All volatile fractions are, or may be returnedto the system at suitable points for cracking with the exception of thelight tar which is used for blending rwith the pitch.

Referring now to the drawing, the numerals I, 2 and 3 designate heatingand cracking coils for the residual oil, light cycle gas oil and heavygas oil respectively. The heating coils are arranged in gas or oil-firedfurnaces. Three separate furnaces are shown on the drawing, although thethree heating coils may also be arranged in the various sections of asingle furnace. The numeral 4 designates a flash drum, numerals 5 and 6reaction chambers and 'I and 3 fractionating towers. The bottoms of thefractionating towers function as tar separators. It will be understoodthat independent tar separators may also be provided for, in which casethey are arranged between the soakingdrums .and the bubble towers. Thetowers 'I and 8 are provided with fractionating means, such as bellcapplates in their upper part and discs and doughnuts in their lower part.Heat exchange coils at the top of the fractionating towers 'I and 8 andlines for returning reflux tothe upper plates of the towers are providedfor, as well known in the art, but are not shown in the drawing. Theseprincipal parts are operatively connectedby various lines and apreferred method of operating the unit is as follows:

The feed stock of residual oil is pumped from a storage tank (not shown)through heating coil I and transfer line I0 into the soaking drum 5. Thesto-ck may be preheated prior to its passage through the heating coil inliquid or vapor heat exchangers (not shown) in various appropriate partsof the unit. The cycle gas oil obtained as redux in towers 'I and 8 ispumped through lines II, I2 and I3 respectively to the heating coil,after which it is united with the heated residual oil and passed throughtransfer line. I0 to the soaking drum 5. Fresh gas oil obtained indistillation of crudes, cr in another cracking operation, may be addedthrough line I4, if desired, The cracking initiated in coils I or 2 orin coil 2 only is continued and carried to the desired extent in thesoaking drum E. The hot cracked eilluent from this drum is thentransferred through line I5 into the 4tower l, from the bottom of whichthe separated heavy tar is removed through line I6 into the ash drumFrom this ash drum all the vaporizable products Vconsisting of heavy gasoil, mostly obtained from the cracking of the residual feed stock, andlight tar mostly synthesized during the cracking of the light cycle gasoil together with some lighter` products, are removed through theoverhead vapor line I'I, while the pitch is discarded through bottomline I8. A further amount of heavy gas oil is obtained as side stream intower 'l and is removed through line I9. The light cycle gas oil iscollected in tower i at a higher level and is returned to coil 2 throughline II as previously described. Still at a further level in tower 'I,the naphtha distillate is obtained as side stream and removed throughline 2! into a distillate storage tank (not shown). Finally the gas ispassed through the overhead line 2i into a gas storage tank (not shown).The contents of vapor line I'l and the side stream line I9 consisting ofheavy gas oil and light tar are then pumped through line 22, heatingcoil 3 and transfer line 23 into the soaking drum t. The crackingconditions in coil 3 and soaking drum ii are maintained strenuous enoughto crack the heavy gas oil but not sufficient to crack the refractorylight tar to a considerable extent. This is the more readily achievableby choosing cracking conditions adequate to crack heavy gas oil to lightgas oil, but not attempting to choose cracking conditions suflicientlydrastic to crack heavy gas oil to any large extent all the way tonaphtha. Of course the lighter conditions which produce predominantlylight gas oil as the cracked product will also produce some naphtha andgas but in smaller amounts. The hot cracked eiiluent is passed throughline 24 into the lower part of tower 8 at the bottom of which a tarcollects, consisting mostly of the light tar. This is removed throughline uxed with a pitch from line I 8 and then passed through line 25through a heat exchanger or cooler (not shown) into the fuel oil storagetank 21. A heavy gas oil which collects in tower 8 as lowest side streamis returned through lines 28 and 22 to the cracking coil 3. A light gasoil is obtained as middle side stream and is returned to cracking coil 2through lines I2 and I3. A naphtha distillate is collected as upper sidestream and removed through line 29 into the distillate storage tank (notshown). Finally the gas is passed through line 30 into the gas storagetank (not shown).

My process may be modied in various ways. Thus, for example, soakingdrums 5 and (i may be omitted altogether and replaced by correspondingsoaking sections, i. e; moderately heated tubes in furnaces I and 3respectively in which the cracking is completed to the desired extent.Other changes and modications will be apparent to those skilled in theart and, therefore, my process is not Yto be limited by the examplegiven for illustration but only by the following claims in which it ismy intention torclaim all novelty inherent in the invention.

What I claim is: Y

1. A method Aof processing hydrocarbon oil to convert higher-boilinghydrocarbons into lowerboiling hydrocarbons suitable for motor fuelwhich comprises subjecting fresh charging oil to cracking conditions oftime and temperature untilrthe desired degree of conversion has beenattained, thereafter passing the cracked products to a separating zoneand separating the same into vapors and residue, fractionating thevapors so separated into a distillate fraction of the desired motor fuelboiling range, a light condensate fraction substantially free of tarryconstituents and a heavy condensate fraction containing tarryconstituents, subjecting said light condensate fraction to cracking in aseparate cracking zone under more drastic temperature conditions thanthat to which said fresh oil is subjected, combining the crackedproducts' from said last-named cracking zone with said firstnamedcracked products prior to passing the same to said separating zone,subjecting said heavy condensate fraction to cracking conditions moremild than that to which'said light condensate fraction is subjected in aseparate cracking zone until the desired conversion has been effected,passing the cracked products from said last-named cracking Zone into aseparating zone independent from said first-named separating zone andseparating the products therein into vapors and residue, fractionatingthe resulting vapors to form a desired distillate fraction within themotor fuel boiling range, a light condensate fraction relatively freeof. tarry constituents and a heavy condensate fraction containing tarryconstituents, combining said, last-named light condensate fraction withsaid first-named light condensate fraction prior to its passage to thecracking zone, combining said last-named heavy condensate fraction Withsaid first-named heavy condensate fraction prior to its passage to thelast-named cracking zone.

2. A'method of processing hydrocarbon oil to produce motor fuel and aresidue suitable for fuel oil, which comprises subjecting fresh chargingstock containing crude constituents to cracking in a cracking zone untilthe desired conversion has been attained, passingfthe cracked productsinto a separating zone, separating the cracked products into a vaporfraction and a liquid residue fraction having a viscosity greater thanthat desired for said fuel oil, fractionating the vapor so separated toseparate the same to a desired dis-` tillate fraction, a lightcondensate fraction boiling above the motor fuel boiling range and aheavy condensate fraction, subjecting the light condensate fraction tocracking ina cracking zone separate from said first named cracking zoneto convert said condensate fraction into lower boiling constituentswithin the motor fuel boiling range, combining cracked products from thesaid last named cracking zone with the cracked products from the saidi'lrst named cracking zone, subject-ing the liquid residue from theinitial separation after the initial 'cracking step to additionaldistillation to vaporize additional constituents therefrom and'produce aliquid residue having a viscosity greater than that desired for fueloil, subjecting the heavy condensate fraction produced from the initialcracking operation and said additional constituents from the liquidresidue of the initial cracking operation to cracking in a cracking zoneseparate from said initial cracking Zone to convert the mixture intolower boiling constituents within the motor fuel boiling range,thereafter passing the cracked products from the last named crackingzone into a separating zone separate from the said first namedseparating zone, separating the cracked products in said separating Zoneinto a vapor fraction and a liquid residue fraction having a viscosityless than that desired for said fuel oil, combining the residue from theadditional distillation of the initial residue `from the said rst namedseparating zone with the residue from the second named separating zoneto produce a fuel oil of desired gravity and fracticnating the vaporsfrom the last named separating Zone to separate a desired distillatefraction and condensate fractions, and returning said condensatefractions to the cracking Zones for the original condensate fractions.

3. A method of processing hydrocarbon oil to convert higher boilinghydrocarbons into lower boiling hydrocarbons suitable for motor Vfueland a residue suitable for fuel oil, which comprises subjecting freshcharging oil to cracking conditions of time and temperature until thedesired degree of conversion has been attained, thereafter passing thecracked products to a separating Zone and separating the same intovapors and residue, fractionating the vapors so separated intoY adistillate fraction of the desired motor fuel boiling range, a lightcondensate fraction substantially free of tarry constituents and a heavycondensate fraction, subjecting said light condensate fraction tocracking in a separate cracking zone under more drastic temperatureconditions than that to which said fresh oil is subjected, combining thecracked products from said last named cracking zone with said firstnamed cracked products, subjecting said heavy condensate fraction tocracking conditions more mild than that to which said light condensateis subjected in a separate cracking Zone until the desired conversionhas been effected, passing the cracked products from said last namedcracking Zone into a separating Zone independent from said first namedseparating Zone and separating the products therein into vapors andresidue, fractionating the resulting vapors to form a desired distillatefraction withoin the motor fuel boiling range, a light condensatefraction relatively free of tarry constituents and a heavy condensatefraction, combining said last named light condensate fraction with saidrst named light condensatefraction prior to the passage to the secondnamed 'cracking Zone, combining said last named heavy condensatefraction with said first named heavy Ycondensate fraction prior to its.passage to the last named cracking zone and combining the residue fromthe first named separating zone with the residue from the second namedseparating zone to produce a fuel oil of `desired gravity.

4. A method. of processing hydrocarbon oil to convert higher boilinghydrocarbons to lower boiling hydrocarbons suitable forv motor fuel anda residue suitable for fuel oil, which comprises passing the freshcharging oil in a stream thru an elongated reactor of restricted crosssectional area under cracking conditions of time and temperature untilthe desired degree of conversion has been attained, thereafter passingthe cracked products to a separating Zone and separating the same intovapors and residue, fractionating the vapors so separated into adistillate fraction of the desired motor fuel boiling range, a lightcondensate fraction substantially free of tarry constituents, and aheavy condensate fraction, subjecting said light condensate fraction tocracking in a separate cracking zone under more drastic temperatureconditions than that to which said fresh oil is subjected, combining thecracked products from said last named cracking zone with said irst namedcracked products, subjecting the liquid residue from the rst namedcracking Zone to additional distillation to vaporize additionalconstituents therefrom and produce a liquid residue having a viscositygreater than that desired for motor fuel, subjecting the heavycondensate fraction and said additional vaporized constituents from thesaid first named liquid residue to cracking under conditions more mildthan that to which said light condensate fraction is subjected in acracking zone separate from previously mentioned cracking zones toconvert the mixture to lower boiling constituents within the motor fuelboiling range, passing the cracked products from said last namedcracking zone into a separating zone independent from said first namedseparating zone and separating the products therein to vapors andresidue, fractionating the resulting vapors to form a desired distillatefraction within the motor fuel range, a light condensate fractionrelatively free of tarry constituents and a heavy condensate fraction,passing said last named light condensate'fraction together with saidfirst named light condensate fraction to the second mentioned crackingzone, and passing said last named heavy condensate fraction with saidrst named heavy condensate fraction to the last named cracking zone.

5. A method of processing hydrocarbon oil to produce motor fuel and aresidue suitable for fuel oil which comprises subjecting fresh chargingstock containing crude constituents to cracking in a cracking zone untilthe desired conversion has been attained, passing the cracked productsinto a separating zone, separatng the cracked products into a vaporfraction and a liquid residue fraction, fractionating the vapor soseparated toi separate the same into a desired distillate fraction,alight condensate fraction boiling above the motor fuel boiling rangeand a heavy condensate fraction, subjecting the liquid residue soseparated to additional distillation to vaporize additional constituentstherefrom and produce a liquid residue having a viscosity greater thanthat desired for motor fuel, subjecting the light condensate fraction tocracking in a cracking zone separate from said rst named cracking zoneto convert said condensate fraction into lower boiling constituentsWithin the motor fuel boiling range, combining cracked products fromsaid last named cracking zone with the cracked products from said firstnamed cracking zone, subjecting the heavy condensate fraction tocracking in a cracking zone independent of said rst and said secondnamed cracking zones until the desired conversion has been attained,thereafter passing the cracked products from the last named crackingzone into a separating zone separate from said first named separatingzone, separating the cracked products in said separating zone into avapor fraction and a liquid residue fraction having a viscosity lessthan that desired for said fuel oil, combining said last named residuewith said second named residue to produce a fuel oil of desired gravityand fractionating the vapors from the last named separating Zone toseparate a desired distillate fraction and a condensate fraction, andreturning said condensate fraction to the cracking zones for theoriginal condensate fractions.

6. A method of processing hydrocarbon oil to produce motor fuel an-d aresidue suitable for fuel oil, which comprises subjecting fresh chargingstock containing crude constituents to cracking in a cracking Zone untilthe desired conversion has been attained, passing the cracked productsinto a separating zone, separating the cracked products into a vaporfraction and a liquid residue fraction, fractionating the vapors soseparated to separate the same to a desired distillate fraction, a lightcondensate fraction boiling above the motor fuel boiling range and aheavy condensate fraction, subjecting said light condensate fraction tocracking in a separate cracking zone under more drastic temperatureconditions than that to which said fresh charging stock is subjected,combining the cracked products from said last named cracking zone withsaid rst named cracked products prior to passing the said first namedcracked products to said separating zone, subjecting the liquid residuefrom said separating zone to an additional distillation to vaporizeadditional constituents therefrom and produce a liquid residue having aviscosity greater than that desired for fuel oil, subjecting the heavycondensate fraction and said additional constituents vaporized from saidfirst named liquid residue to cracking under conditions more mild thanthat to which said light condensate fraction is subjected in a crackingzone separate from said previously named cracking zones to. convert themixture into lower boiling constituents Within the motor fuel boilingrange, thereafter passing the cracked products from the last namedcracking zone to a separating zone separate from said first namedseparating zone, separating the cracked products in said separating zoneinto a vapor fraction and a liquid residue fraction having a viscosityless than that desired for said fuel oil, combining said last namedresidue with the residue obtained from the additional distillation ofthe initial residue to produce a fuel oil of desired gravity,fractionating the vapors from the last named separating zone to separatea desired distillate fraction Within the motor fuel boiling range, alight 'condensate fraction relatively free of tarry constituents and aheavy condensate fraction, combining said last named light condensatefraction with said rst named light condensate fraction prior to itspassage to the cracking zone an-d combining said last named heavycondensate fraction with said rst named heavy condensate fraction priorto its passage to the last named cracking Zone.

WARREN K. LEWIS.

